In conventional calendered papermaking for providing papers used in printing, a fibrous web may prepared from an aqueous solids mixture which may comprise wood pulp and/or synthetic fibers along with various additives such as sizing agents, binders, fillers, pigments, etc. Sizing agents are used primarily to prevent excess penetration, wicking, spreading, resistance to blotting etc., of water or ink, and especially internal absorption of the water or ink by the resulting paper substrate. The sized paper substrate may exhibit improved properties in terms of, for example, print density, because more of the dye or pigment present in the ink remains on the surface of the paper substrate, rather than being absorbed internally by the paper substrate.
There are two basic methods for sizing paper substrates: internal paper sizing and surface paper sizing. In internal paper sizing, an internal sizing agent is included, added, etc., during the papermaking process before the fibrous paper substrate is formed. These internal sizing agents may include rosin soap sizes, rosin emulsion sizes, alkyl ketene dimers, alkenyl succinic anhydrides, etc. Internal sizing agents may also be referred to as “reactive sizing” agents due to reactions of these internal sizing agents which occur during the papermaking process to enable the molecules of the internal sizing agent to be retained, anchored, incorporated, oriented, etc., within or by the paper substrate.
In surface paper sizing, a surface sizing agent is coated on, applied to, etc., one, or more often both surfaces of the already formed paper substrate, often in the form of a sizing solution comprising the surface sizing agent. The most common surface sizing agent used in such solutions is starch which may be either in a cooked or modified form (e.g., oxidized or enzyme converted). This surface starch sizing solution may be applied to the surface(s) of the paper substrate by using a device called a size press. A conventional size press comprises two adjacent and cooperating applicator rolls which are fairly close together to define a nip and may be in a vertical, horizontal, inclined, etc., configuration.
During the size press operation, the paper substrate to be sized is fed through the nip between the two applicator rolls of the size press. The surface starch sizing solution may be sprayed towards the nip of the size press to create a flooded or “pond” region of the sizing solution which then coats, is absorbed onto, etc., the surface(s) of the paper substrate as the paper substrate passes through the nip. In the case of a metering size press, the paper surface sizing composition may be transferred from paper surface sizing composition supply reservoirs by take up rolls to the applicator rolls. The amount of paper surface sizing composition transferred to the applicator rolls may be controlled by a metering rod which spreads the paper surface sizing composition before being applied to the surface(s) of the paper substrate.
In addition to size presses, the surface starch sizing solution may also be applied to the surface(s) of the paper substrate by other methods or devices, such as by dip coating, slot extrusion, etc. In dip coating, the paper substrate may be transported below the surface of the starch sizing solution by a single roll such that the surface(s) of the paper substrate are saturated, followed by removal of any excess starch sizing solution by passing the saturated paper substrate through squeeze rolls and then drying (e.g., by an air dryer). In slot extrusion, the paper substrate may be passed through die lips of a flat die in close proximity to provide a continuous film of starch sizing solution evenly distributed across one surface of the paper substrate, followed by drying (e.g., with an air dryer), and then repeating the slot extrusion operation to treat the other surface of the paper substrate with the starch sizing solution.
In recent years, the use of ink-jet printing methods has been increasing at a rapid rate. Ink jet printing is a method for forming ink images on a paper substrate from deposited droplets of ink comprising dyes or pigments. This printing method enables high-speed and full-color printing to be achieved. In ink jet printing, the fine droplets of ink are sprayed or jetted from printing nozzles at a high speed so as to direct the ink droplets toward, and deposit these droplets on, the paper substrate to provide printed images on the paper substrate.
The ink used in ink jet printing may contain a large quantity of solvent. In the case of inks comprising pigments, the ink may also be in the form of a pigment emulsion. The presence of large quantities of solvent in the ink, or the use of pigment emulsions in the ink, may increase the dry time for the ink droplets deposited on the surface of the paper substrate, and may thus lead to, for example, smearing of the deposited ink droplets. Ink dry time may particularly increase when the ink droplets are deposited onto the surface(s) of a paper substrate which has been treated with an internal and/or surface sizing agents.
To decrease dry time of internal/surface sized paper substrates during ink jet printing, the paper substrate may be treated with a drying agent. These drying agents may include monovalent metal drying salts (e.g., sodium chloride, etc.) or multivalent metal drying salts (e.g., calcium chloride, magnesium chloride, aluminum chloride, etc.) The drying agent is often applied to the surface(s) of a paper substrate at the same time as the surface sizing solution, but may also be applied after the surface sizing solution is applied to the surface(s) of a paper substrate. The drying agent may be incorporated in the surface starch sizing solution which is then applied to the surface(s) of a paper substrate (e.g., by the “pond” at the nip of a size press) or may be applied to the surface(s) of the paper substrate separately from the surface starch sizing solution. The presence of these metal drying salts on the paper substrate surface destabilizes the pigment emulsion in the ink droplets deposited by an ink jet printer. The resulting destabilization of the pigment emulsion causes precipitation of pigment particles, thus resulting in a faster and improved dry time.